1. Field of the Invention
This invention relates to the use of partial esters of citric acid and of mixtures of citric acid partial esters for the selective liquid/liquid extraction of iron from aqueous acidic solutions.
2. Statement of Related Art
The recovery of zinc metal suitable for technical purposes is carried out in by far the majority of cases by hydrometallurgical methods. The recovery process is normally carried out by desulfurizing the crude ores containing zinc and, in smaller quantities, other metals by roasting. The metal oxides left after roasting are dissolved hydrometallurgically with dilute sulfuric acid. Maximum recovery of all the valuable metals present in the crude ore (copper and cadmium besides zinc) requires digestion conditions under which iron is co-dissolved. However, since iron ions interfere with the electrolytic recovery of zinc, they have to be removed from the zinc sulfate solution before the zinc electrolysis process. In the industrial processes used at the present time, iron is precipitated from electrolysis solutions such as these. In these processes, the iron is precipitated as jarosite, as goethite, or as hematite (cf. "Ullman's Enzyklopadie der technischen Chemie", 4th Edition, Vol. 24, pages 601 et seq. and Winnacker-Kuchler, "Chemische Technologie", 3rd Edition, Vol. 6 (Metallurgie), pages 306 et seq.). After filtration, iron-containing precipitates such as these accumulate as filter cakes of high water content (approx 50%) and, depending on the quantity of heavy metal impurities present therein, have to be dumped at special dumps in accordance with legal and environmental requirements. Experience has shown that, for every metric ton of zinc metal produced hydrometallurgically, approximately 1 metric ton of iron-containing sludge has to be dumped.
Where the iron is precipitated as hematite, the product obtained contains no heavy metals other than iron. According to A. v. Ropenack, "Erzemetall" 35, 534 (1982), this product may be used in the production of steel and building materials. The disadvantage of precipitating iron by the "hematite process" lies in the cost of the process. The precipitation is carried out with pure oxygen at temperatures of at least 180.degree. C. under a pressure of 15 bar.
Apart from the possibility of reusing the iron residues which only the "hematite process" affords, a search is being made on ecological grounds for processes in which few, if any, heavy-metal-bearing residues to be deposited on dumps are formed and in which the iron-containing fractions of the electrolyte solutions can be reprocessed. For some time, an alternaive to the precipitation processes discussed above has existed in liquid/liquid extraction processes by which the iron can be removed from zinc-containing electrolyte solutions and subsequently recovered. However, most of the processes suitable for the liquid/liquid extraction of iron from aqueous electrolyte solutions could not be implemented in practice, i.e. on an industrial scale, either due to the inadequate selectivity of metal separation or due to the inadequate chargeability of the organic phase at low pH values. The so-called "Espindesa" process is mainly used in practice (cf. A. J. Monhemius, "Topics in Non-Ferrous Extractive Metallurgy", R. Burkin (Ed.), pages 104 et seq; G. Thorsen, "Handbook of Solvent Extraction", T. C. Lo et al., 1983, pages 709 et seq.). In this process, an aqueous solution obtained from the leaching of ore with hydrochloric acid is first extracted with a secondary amine, to separate off the metals zinc, copper, cadmium and iron as chloro complexes. After a re-extraction stage with water, Zn.sup.2+ and Fe.sup.3+ are extracted with di-(2-ethylhexylphosphoric acid) (D-2-EHPA) in a following process step. First zinc and then iron may be separated off from the resulting solutions in two successive circuits; the two metals are then separately worked up in conventional extraction processes.
The disadvantage of the Espindesa process is that it is complicated by the multistep procedure involved both in the extraction and in the re-extraction using different extraction reagents. Accordingly, there has long been a need to provide new processes for the liquid/liquid extraction of iron from aqueous acidic solutions which do not have any of the above-described disadvantages of the prior art.
In addition, a process for the extraction of iron from zinccontaining aqueous electrolyte solutions using so-called "Versatic acids" is known from A. J. van der Zeeuw, "Hydrometallurgy" 2, 275 (1976) and from German Pat. No. 24 04 185. Before the extraction step, the organic phase containing Versatic acids is treated with roasted ore ("neutral leaching"), the zinc salt of the corresponding Versatic acid being formed. In the following extraction step, the aqueous iron-rich solution obtained from the digestion of the neutral leaching residues is brought into contact with the zinc-laden organic phase, Zn.sup.2+ being exchanged for Fe.sup.3+ in the Versatic acid salts. A considerable limitation is imposed by the fact that Versatic acids only begin to extract iron from the aqueous solutions at a pH value of 1.7, reaching their maximum load at a pH value of 2.6. This requires a considerable, undesirable reduction in the pH value which seriously restricts the scope of application of this extraction process.